Conventional internal combustion engines having a V-, W-or flat (i.e., horizontally opposed) shaped configurations include multiple cylinder banks. The cylinder banks include pistons that are reciprocally driven through cylinders by a combustion process to produce driving force. An air and fuel mixture is provided and is ignited within the cylinders during the combustion process. The air and fuel quantity within a cylinder defines the work output of the cylinder. The air rates of the cylinders are controlled by the phase angle or timing of a camshaft with respect to a driven crankshaft. The fuel rate is controlled by the pulse-width of a fuel injector.
The timing of intake valve closing with respect to piston position within the cylinder influences the volume of air that is drawn into the cylinder. When intake valve closing occurs near a bottom-dead-center (BDC) piston position, cylinder volume is changing slowly and variations in intake valve timing have only a minor effect. If an engine having variable cam timing implements an early or late intake valve closing strategy to improve engine efficiency, intake valve closing can occur when the piston velocity is higher and air volume into the cylinder is changing rapidly. Differences in the intake valve closing timing (i.e., cam position of the camshafts) can significantly influence the volume of air drawn into the cylinder.
Conventional control algorithms attempt to balance the bank-to-bank cam positions of the camshafts. This is achieved by measuring the radial position of mechanical targets installed on each camshaft or cam phasers associated with each camshaft. Balancing of the bank-to-bank cam positions, however, does not insure bank-to-bank balancing of intake air flow. This is a result of manufacturing and assembly variations that create ambiguity between sensed cam positions and actual timing of intake valves.
Imbalance of intake air flow results in an A/F ratio imbalance across the cylinder banks that effects engine smoothness and engine efficiency. Traditionally, the fuel rate is trimmed to compensate for air flow variation across the cylinder banks. However, this compensation strategy fails to correct the fundamental problem of air flow imbalance. Another method of alleviating this imbalance is to provide tighter manufacturing and assembly tolerances. This, however, results in increased manufacturing and assembly costs.